To reduce NOx emissions a selective catalytic reduction (SCR) catalyst may be used in an exhaust gas discharge system. A reductant is injected upstream of the SCR catalyst. The NOx will react with the reductant, or the reductants products, in the SCR catalyst to create byproducts such as nitrogen and water.
One example approach is to inject the reductant downstream of a turbine on a turbocharger and upstream of the SCR catalyst. Specifically, the reductant is injected into exhaust flow coming out of the turbo. Another approach is to inject the reductant upstream of both the turbocharger and the SCR catalyst. As a reductant, ammonia NH3 and urea are commonly used. Further, use may be made of HC enrichment with unburned hydrocarbons being introduced directly into the exhaust gas discharge system.
One of the problems with the above approaches recognized by the inventors is temperature regulation in the SCR catalyst. At low temperatures the NOx may not be reacted and released to the atmosphere. Close coupling of the exhaust gas aftertreatment systems and the SCR catalyst can lead to high temperatures which release ammonia and NOx without reduction. Another problem recognized is the exhaust gas flow rate is only dependent on the engine operating conditions and cannot be controlled to promote better decomposition of the injected reductant. Further to decompose urea to ammonia requires a narrow temperature range of the exhaust gases. Another problem is with introducing unburned hydrocarbons directly which requires extra fuel thereby increasing fuel consumption.
One solution relates to an internal combustion engine comprising an intake system for the supply of charge air and an exhaust-gas discharge system for the discharge of the exhaust gases. Wherein the exhaust-gas discharge system further comprises at least one selective catalytic converter arranged in the exhaust-gas discharge system, which serves for the reduction of nitrogen oxides, and an oxidation catalytic converter being arranged, as a further exhaust-gas aftertreatment system, in the exhaust-gas discharge system upstream of the at least one selective catalytic converter. Further a bypass line branches off from the exhaust-gas discharge system upstream of the oxidation catalytic converter and issues into the exhaust-gas discharge system again between the oxidation catalytic converter and the at least one selective catalytic converter; and a dosing device being provided for introducing liquid urea as a reducing agent for the at least one selective catalytic converter into the bypass line. The virtue of low mass flow rates of the overall exhaust gas flow in the bypass line 8 as opposed to the main exhaust line promotes longer residence time and better decomposition of the injected reductant.
Another solution relates to a method for controlling an engine having a turbocharger with a turbine positioned in the engine exhaust and a selective catalytic reducer (SCR) positioned downstream of the turbine comprising injecting a reductant into the SCR and controlling the temperature of the reductant by portioning an exhaust flow into said reductant between a portion of engine exhaust from upstream of the turbine and another portion of engine exhaust from downstream of the turbine.
A method for controlling nitrogen oxide emissions from an engine having a turbocharger with a turbine positioned in the engine exhaust and a selective catalytic reducer (SCR) positioned downstream of the turbine comprising controlling a wastegate valve positioned on a first bypass line segment between the engine exhaust and the turbine to divert a portion of exhaust gases from the turbine to maintain a desired engine torque. Further, controlling injection of urea into the SCR through a urea dosing element positioned in an exhaust as dosing line coupled to the SCR, said exhaust gas dosing line receiving exhaust flow from said first bypass line and receiving exhaust flow from a second bypass line positioned downstream of the turbine exhaust and controlling the temperature of said urea dosing element by controlling a control element coupled to said first and said second bypass lines to portion exhaust flow into said exhaust gas dosing line between exhaust flow from said first bypass line and exhaust flow from said second bypass line.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.